Personal digital assistants, PDAs, have become necessities for businessman. A PDA can have functions such as managing personal data, managing a personal schedule, storing data, giving schedule reminders and so on. However, such management functions gradually are becoming insufficient, especially for those who require a larger memory space to store data or need to renew application programs often. Moreover, the increased complexity of the operation system and the application program also increase the need for a PDA with high storage capacity.
Typical PDA designs include an expansion port for receiving an expansion card to expand the storage capacity and to increase the renewable functions thereof. Such expansion card is a memory card for expanding the memory space or an application program card for providing additional application software. However, the single expansion port is not enough for a user. For example, if an application program card is inserted into the single port to provide additional software, an additional memory space cannot be provided by this PDA. Obviously, the single port design cannot satisfy all users. Therefore, a PDA with dual ports has been developed by engineers.
Developing the PDA with dual ports gives rise to other problems. For example, when two expansion cards are respectively inserted into the dual expansion ports, how to recognize the exact expansion port to access the exact data becomes a challenge. Typically, the problem can be solved by either using an additional address line to create an additional register to store the status of the two ports or dividing the register of the original signal port to respectively store the status of the two ports. However, both such methods require an additional address line, which causes the register address to be different from that of the single port design. Therefore, the typical PDA with dual ports requires a new driver design. On the other hand, the additional address line also increases the complexity of the circuit.